1. Field of the Invention
The present invention relates to a belt conveyor unit having an endless belt that is stretched by a plurality of stretching rollers and is moved around the stretching rollers and an electrophotographic image forming apparatus, such as a printer or a copying machine, including the belt conveyor unit.
2. Description of the Related Art
Some existing electrophotographic printers and copying machines include a belt conveyor unit having an endless belt that is stretched by a plurality of stretching rollers and is moved around the stretching rollers. Such a belt conveyor unit has the following disadvantage. That is, when a belt moves, the belt may be shifted to one side in the width direction (i.e., a direction substantially perpendicular to the belt moving direction).
To solve such a problem, Japanese Patent Laid-Open No. 2005-31504 describes a structure in which a belt has an inner peripheral surface having a rib thereon as a protruding portion and each of stretching rollers has a flange at the end. According to Japanese Patent Laid-Open No. 2005-31504, if the belt is shifted, the flange is brought into contact with the rib to regulates the position of the rib so that the lateral shift of the belt can be regulated.
The drawback of the structure described in Japanese Patent Laid-Open No. 2005-31504 is that it requires the rib that prevents the lateral shift of the belt and, if the rib is bonded to the inner peripheral surface of the belt, it requires a rib bonding step when producing the belt. In the case where a rib is not provided, an edge portion of the belt in the width direction can be in direct contact with the flange. However, in such a case, there is a risk of cracking of the belt from the edge of the belt. Thus, the durability may be decreased.